This invention relates to an electrolytic capacitor utilizing an anodized porous tantalum pellet as anode and an azeotrope of hydrochloric acid and water as electrolyte. The azeotrope electrolyte preferably contains about 20 wt% hydrochloric acid.
Strong acid electrolytes, notably sulfuric acid, have been used in tantalum pellet capacitors as have lithium chloride electrolytes. However, improvements in capacitor operation, particularly at -55.degree. C., are desirable and, in particular, improvements in equivalent series resistance, ESR, and impedance, Z.
Hydrochloric acid has been mentioned as a strong acid electrolyte. However, it was believed that it could not be used with silver cases as there was already a problem with silver "creep", i.e., dissolution of silver followed by replating and dendritic silver growth which bridged the space between anode and cathode, shorting out the capacitor. Silver salts are frequently dissolved in current electrolytes to saturate the electrolyte and prevent dendritic silver growth. It was also believed from the listed freezing point of aqueous hydrochloric acid that it would be an unsuitable electrolyte for -55.degree. C. operation.